Maximal skin wettedness as a function of environment and metabolic rate in unacclimated young and older adults (PSU HEAT Project)

Abstract

Maximal skin wettedness (ω max ), the proportion of the skin covered in sweat at the upper limits of compensable heat stress, is an important parameter for modeling human heat stress responses. We previously determined ω max in two extreme environments during activities of daily living across the life span; however, ω max has yet to be quantified across a broader range of environments and metabolic rates for young (Y) and older (O) adults in extreme heat. The present study used partitional calorimetry to determine ω max across a wide range of hot environments (34–49 °C dry-bulb temperature, T db ; 14–80% relative humidity, rh) in 51 Y (18–35 yrs; 29 F) and 55 O (65–92 yrs; 33 F) during minimal activity (MinAct; ~150 W), light ambulation (LightAmb; ~250 W; Y only), and rest (O only; ~90 W). During MinAct, ω max was higher in Y compared to O across environments (all P ≤ 0.008) and ranged from 0.43 to 0.99 as humidity increased in Y and 0.21 to 0.83 in O. During LightAmb in Y, ω max ranged from 0.53 to 1.10 but was higher compared to MinAct only in hot-dry environments ( P < 0.0001). At rest in O, ω max ranged from 0.16 to 0.78 and was lower compared to MinAct only in a 53–60% rh condition (36 °C, T db ) ( P < 0.008). These findings indicate that ω max varies with age, metabolic rate, and environment. ω max established herein for unacclimated young and older adults across environments and relatively low metabolic rates can be used for heat stress modeling in these populations and environments.

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